In the anime there is an episode where Sabrina shrinks Ash, exactly how small does she shrink him, as in 1 inch 2.3 inches what size exactly does he become(just Ash, not Brock or Misty)? I've seen things that says he is shrunk, but the closest to a specific size is doll size, which doesn't help since dolls come in different sizes.
If we use ball-joint dolls as an example, then the bigger ones are 60 cm (24 inches) and the smaller ones are 40 cm (15.5 inches). So maybe pick somewhere between those and you should be fine.